Thermal dye sublimation transfer also called thermal dye diffusion transfer is a recording method in which a dye-donor element provided with a dye layer containing sublimable dyes having heat transferability is brought into contact with a receiver sheet and selectively, in accordance with a pattern information signal, heated with a thermal printing head provided with a plurality of juxtaposed heat-generating resistors, whereby dye from the selectively heated regions of the dye-donor element is transferred to the receiver sheet and forms a pattern thereon, the shape and density of which is in accordance with the pattern and intensity of heat applied to the dye-donor element.
A dye-donor element for use according to thermal dye sublimation transfer usually comprises a very thin support e.g. a polyester support, one side of which is covered with a dye layer, which contains the printing dyes. Usually an adhesive or subbing layer is provided between the support and the dye layer. Normally the opposite side is covered with a slipping layer that provides a lubricated surface against which the thermal printing head can pass without suffering abrasion. An adhesive layer may be provided between the support and the slipping layer.
A dye-image receiving element for use according to thermal dye sublimation transfer usually comprises a support, e.g. paper or a transparant film, coated with a dye-image receiving layer, into which the dye can diffuse more readily. An adhesive layer may be provided between the support and the receiving layer. On top of said receiving layer a separate release layer may be provided to improve the releasability of the receiving element from the donor element after transfer is effected.
The dye layer can be a monochrome dye layer or it may comprise sequential repeating areas of different colored dyes like e.g. of cyan, magenta, yellow and optionally black hue. When a dye-donor element containing three or more primary color dyes is used, a multicolor image can be obtained by sequentially performing the dye transfer process steps for each color.
One of the possible applications of thermal dye sublimation transfer printing is the production of hard copies of medical diagnostic images. Although such a hard copy can be produced on a reflective support such as paper, in most applications hard copies are produced on transparancies. Depending on the particular application these hard copies can be monochrome, in particular monochrome black, or multicolored.
Hard copies of medical diagnostic images produced on transparancies are read from the front of a light box through a light source.
A disadvantage of the known thermal dye sublimation transfer recording materials for producing hard copies of medical diagnostic images is that when the image produced on the thermal dye sublimation transparancy is observed from the front of a light box, small angle reflections from the near surroundings (e.g. the observer seeing himself as in a mirror, reflections from a light source approximately behind the observer) make it difficult to interpret the actual image, especially in the high density areas and hence disturb the diagnosis.